A multimeter, as the name suggests, is an instrument which is capable of measuring a number of different parameters, such as voltage, current, resistance, and the frequency of an input signal. Modern digital multimeters (DMMs) have become quite advanced and are also sometimes capable of measuring capacitance, duty factor, and temperature. It is common for DMMs to be used for making measurements at remote locations which do not provide ready access to an AC power line, and for this reason, multimeters typically are battery powered. These remote locations could be almost anywhere, for example, at the top of a telephone pole, inside a sewer drainpipe, or at the top of a mountain.
It is very irritating to a user of a DMM to get to one of these locations, prepare to make a measurement, and then discover that the batteries which power the DMM are very weak or dead. To alleviate this annoyance, many premium DMMs include LOW BATTERY indicators. Typically, these indicators tend to light up when the battery has only just a few minutes of battery life remaining. Unfortunately, a non-illuminated LOW BATTERY warning indicator actually gives a false indication of good battery condition when the battery voltage is slightly above the warning light set point. In such a case the user might think that the battery was good, only to have the LOW BATTERY warning indicator illuminate in response to the user turning on the back light for his display. This occurs because the back light, when on, generally uses about ten times the current normally drawn by the rest of the DMM when the back light is off. Thus, the use of the backlight tends to load the weak battery, and causing its terminal voltage to drop even faster, and thereby causes the LOW BATTERY indicator to illuminate. In such a case the user may note that the battery appears good before climbing down into a sewer to make a measurement. He would then be unpleasantly surprised, upon lighting the DMM backlight in preparation for performing the measurement, to find that the LOW BATTERY warning indicator is now illuminated.
From the above example, it seems clear that relying upon a single threshold voltage level to determine battery life is not an optimum solution for the process. An alternative solution is the one chosen by battery indicators found in the camcorder and laptop computer arts. This solution to the problem uses a bar graph (thermometer-type) depiction of battery life. The bar graph is driven in response to the battery voltage crossing a series of thresholds. Unfortunately, the graphs tend to be nonlinear. That is, the graphs do not convey that the time to discharge the battery through the last quarter of the graph is very much shorter than the time to discharge through the first quarter of the graph, due to battery discharge characteristics. This uncertainty is compounded by the fact that the user does not know the value of an individual segment of the bar graph.
One might think that it is a simple matter for a voltage measuring device to measure its own battery voltage. Unfortunately, this is not true, for several reasons. Modern DMMs are precision instruments employing highly sophisticated Analog to Digital Converters (A/Ds). These A/Ds are highly accurate in the center of their dynamic range but tend to be less accurate as the input voltage being measured approaches a respective power supply rail of the A/D converter. The battery voltage powering the DMM is at least a large portion of the power supply rail voltage of the DMM (even if a DC to DC Boost circuit is employed to expand the dynamic range of the A/D converter). Moreover, the input A/D converter is expected to measure both positive and negative voltages. In order to accomplish this task, the input A/D converter is biased to the midpoint of the battery supply voltage (which is essentially the same as operating from a bipolar supply wherein each half of the bipolar supply is equal to half the total battery voltage). The result is that the power supply voltage for the input A/D converter is outside the usable range of the input A/D converter. Thus, as strange as it seems, the DMM cannot use its own measuring circuitry to measure its own power supply voltage.
What is needed is a multimeter battery-state indicator which overcomes the above noted difficulties.